Nobody knows how the whole system works
(news.ycombinator.com)
1.
2.
Old Stockholm Telephone Tower
(news.ycombinator.com)
3.
History of Telecommunications T-Carrier
(news.ycombinator.com)
4.
Passive Microwave Repeaters
(news.ycombinator.com)
5.
2-4 wire converters / hybrids (2009)
(news.ycombinator.com)